This disclosure relates to fluidized bed reactor systems for producing polycrystalline silicon. Polycrystalline silicon is a raw material used to produce many commercial products including, for example, integrated circuits and photovoltaic (i.e., solar) cells. Polycrystalline silicon is typically produced by a chemical vapor deposition mechanism in which silicon is deposited from a thermally decomposable silicon compound onto silicon seed particles in a fluidized bed reactor. These seed particles continuously grow in size until they exit the reactor as polycrystalline silicon particles product. Suitable decomposable silicon compounds include, for example, silane and halosilanes (e.g., trichlorosilane).
Polycrystalline seed particles may be added to the core bed to initiate deposition of silicon. A variety of reactions may take place in the core bed. Silicon deposits from silane onto a silicon particle, resulting in the particle growing in size. As the reaction progresses, silicon particles grow to a desired size and are removed from the core bed and new seed particles are added to the core bed.
Polydispersity is ubiquitous in flows involving solids, and such systems are known to exhibit different behaviors than monodisperse systems. Mixing and heat transfer characteristics change as particle size distributions evolve, and thus the effect of polydispersity on temperature gradients in fluidized bed reactors is of practical importance.
In gas-solid fluidized bed reactors, temperature is a critical parameter that contributes toward reactor performance. Temperature not only affects reaction kinetics, but affects the dynamics of the gas-solid system as well because of the effect on gas density and gas viscosity. Typically in commercial fluidized bed reactors, gas flow rates, freeboard pressures and power supply are controlled, but the actual temperature gradient within the bed is unmonitored.
Inevitably, temperature variations within the bed, especially axially, exist in fluidization applications, such as polysilicon reactors. Thus, temperature set-points based on a thermocouple strategically placed along the reactor wall is often not a good predictor of temperatures in other (axial) regions.
There remains a need, therefore, to better understand the impact of evolving particle size distributions on temperature profiles in fluidized bed reactors to allow for better control of such systems and to prolong the lifespan of reactors by alleviating hot spots within them. That is, there remains a need to recognize and understand the relationship between the width of the particle size distribution of granules in the bed reactor and the temperature in the reactor.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.